A conventional AC-to-AC power converter (also referred to simply as power converter or converter) may be used to adapt a frequency of an electrical power signal to a frequency of a utility grid in order to allow consumers to use the electric AC signal to drive the consumer devices requiring a fixed frequency.
Traditionally, the requirement for a full scale converters and partial converters (DFIG) for a wind turbine has been addressed by connecting together two separate 3 phase inverter modules with further separate modules as necessary for any dynamic braking duty to form an overall power converter with the appropriate ratings. There are many commercial examples of this:
Converteam MV3000 DELTA Module system which allows up to 12 modules to be associated into 1 complete power converter scheme, up to 6 paralleled for generator (or motor) inverter function and up to 6 paralleled for network inverter function. ABB ACS800 Modular power converter system also provides this type of structure.
Typically, such products when applied to systems with power ratings above the rating of 1 module achieve the higher rating by connecting multiple modules together into a parallel array. It is also known that higher power rated systems can be realized by breaking down the overall power processing into a number of separate channels such that higher levels of availability can be achieved in the event of a failure in 1 inverter module, albeit with a reduction in maximum power throughput capability in the partial operating mode. By connecting up separate 3 phase inverter modules in this manner, the dc current has to be carried from one inverter module to another. For a full scale converter the nominal dc current is the result of dividing the turbine rated power by the rated or nominal operational dc link voltage.
In former times, Converteam offered an integrated inverter package ‘Alspa GD4000’ which comprised machine (generator or motor) 3 phase inverter and 3 phase network inverter into 1 package up to a rating of 160 A. These integrated inverter packages were not configured for parallel configuration or sold for such parallel arrangements. Converteam also offered a liquid cooled product integrating 3 separate 3 phase inverter sections into 1 liquid cooled module. This used 1 coolant system (equivalent to 1 heatsink) and 1 dc link laminate.
There may be a need for a power converter which is simplified in its construction and which is more cost-effective. There may be a need for a structure for a power converter module aimed specifically at the requirements for a wind turbine application: A 4Q AC to AC power converter where the intermediate ‘dc’ interface is not a prime port to which interface is required by anything else other than the two inverter sections forming the overall function. In a full scale converter system for a wind turbine, the overall power converter is essentially a frequency changer, receiving variable frequency variable voltage from the generator and applying this to the ac phase terminals of a first inverter stage (operating generally in (active) rectification mode), the dc port (output) of which is connected to the dc port (input) of a second inverter stage (operating generally in an inverting mode) whose ac phase (output) terminals are connected via filtering components as necessary to produce nominally fixed frequency and fixed voltage output to match to a utility grid. For typical generators used in wind turbine applications (for example, permanent magnet direct drive generators) when power throughput has to be curtailed due to loss of network (grid fault for example) then means must be included in the power converter to allow connection to be maintained to provide secondary services such as reactive power support. As a result, means must be included to limit the overvoltage of the dc link capacitor and so stay below the overvoltage trip threshold of the protection circuits. A known circuit that can provide this function is a dynamic brake unit. In conventional drives this is dimensioned to cope with the energy stored in the mechanics of the system, however in this application the dimensioning is determined by that necessary to cope with the energy released from the generator during the time that power is being reduced from prevailing power to zero. This is why we describe this as a ‘dc link overvoltage clamp’ rather than a dynamic brake. From a circuit perspective, both are equivalent, it is only the dimensioning of the associated power components and particularly the energy dissipating resistor element that is different.
The purpose of the invention may be to minimize or eliminate wherever possible material content, components and component over-rating all with the purpose of reducing the cost of the resultant power converter module or power converter system.